A circuit for, for example, increasing the interlace factor from non-interlace with the ratio 1:1 to a single interlace with the ratio 2:1 and multi interlace with the ratios 3:1, 4:1 etc. can be realised by grouping television line rasters in groups of two, three, four rasters etc; for the ratio 2:1 the field synchronizing pulse edge which effects a trigger action is shifted half a line period in the second raster, for the ratio 3:1 the trigger pulse edge is shifted in the second and third rasters, respectively, one-third and two-thirds line period, respectively, for the ratio 4:1 a trigger pulse edge is shifted one-fourth, one-half and three-fourths line period, etc. When no interlace is effected, a displayed picture consists of one sole line raster and for the ratios 2:1, 3:1, 4:1 etc. two, three, four etc. interlaced line rasters make a picture.
Single interlace with the ratio 2:1 is fixed in television standards which apply to public television broadcasting systems. This interlace is prescribed to minimize flicker of the displayed picture and to obtain an improved definition as compared with non-interlace.
For television-film (telecine), television-photograph and television-slide converting systems it is advantageous to use multi-interlace to reduce the perceptibility of the television line structure, for a line number per raster which is equal to or higher than the line number of the standards. When, for example, X-ray television pictures are photographed an interlace factor of 4:1 appears to improve the quality of the photograph to a considerable extent as compared with the ratio 2:1. Further, multi-interlace can be used with the above advantage when pictures are displayed by means of long-persistence display tubes.
The interlace factor can be increased by means of the said circuits for shifting trigger pulse edges in the field synchronizing signal. Such a pulse edge shift can be realized by means of delay devices in the form of, for example, delay lines, multivibrator circuits, pulse counting circuits, shift registers etc. Logic gates may be used for signal combination purposes. Even when the specific construction of the field synchronizing circuit with the changeable interlace factor is left out of consideration, the field synchronizing circuit will be of a complicated structure if no special measures are taken.